This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-094792, filed Apr. 1, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a coloring material, a coloring material for a color filter, a display device and a liquid crystal display device.
In the conventional liquid crystal display device, a color filter is arranged in general on the outside of a glass substrate of the liquid crystal cell or on the inside of a counter electrode of the array substrate. Such a construction requires many manufacturing steps, making it difficult to lower the manufacturing cost.
On the other hand, attentions are paid to a CF on-array in which a color filter (hereinafter referred to as xe2x80x9cCFxe2x80x9d) is arranged on the inside of the array substrate. The CF on-array permits diminishing the manufacturing steps, making it possible to manufacture a liquid crystal display device with a low manufacturing cost. However, since the color filter is in direct contact with the liquid crystal layer, a serious problem is generated as follows.
Specifically, TFT (thin film transistor) is used mainly for driving a liquid crystal display device. It should be noted that, in order to achieve such a TFT driving, it is necessary for the liquid crystal layer to have a large electrical resistance and, thus, a fluorine-based liquid crystal material meeting the particular requirement is mainly used. However, even if in the case of using such a fluorine-based liquid crystal material, the ionic impurities that lower the resistance value are migrated from the CF into the liquid crystal layer. Where the CF is arranged inside the liquid crystal cell, i.e., where the CF is arranged in direct contact with the liquid crystal layer, the impurities of the pigment, the impurities generated by deterioration of the pigment, etc. are dissolved in the liquid crystal layer so as to cause the resistance value of the liquid crystal layer to be lowered.
Particularly, in the CF on-array system in which the CF is arranged on the side of the array substrate, the CF is in direct contact with the liquid crystal layer because the transparent electrode on the CF is patterned, leading to a more serious situation.
In order to overcome the particular problem, it is proposed to form a protective layer consisting of a polymer on the CF positioned on the array substrate. However, additional process steps for forming the protective film are required, leading to an increase in the manufacturing cost.
It is naturally desirable to prevent the impurities that lower the resistance value from being dissolved in the liquid crystal layer without relying on a protective film. For achieving the particular system, it is very important to develop a pigment that is not decomposed by heat, light, etc. and that does not generate such impurities as to lower the resistance value even if decomposed. However, the conventional pigment fails to meet these requirements and, thus, a CF on-array has not yet been realized.
The conventional pigment includes, for example, phthalocyanine green (CI:G7) used in a green layer of the CF. G7, which has a high durability, has been widely used as a pigment for CF. However, where G7 is used in the CF on-array system, various defects, which are considered to be derived from reduction in the resistance value, are generated in the display device.
These defects are caused by the impurity generation. Although only traces of impurities are generated, the reduction in the resistance value of the liquid crystal layer is markedly affected by these impurities.
In order to achieve a CF on-array system by overcoming the above-noted problem, it is necessary to develop a pigment for CF having the characteristics given below:
1) The pigment should be stable against alkaline conditions, heat and light and should be excellent in durability;
2) Even if decomposed, the decomposed substances of the pigment do not form ionic impurities that cause the resistance value of the liquid crystal material to be lowered; and
3) The pigment should have a spectrum suitable for use in a CF so as to be capable of achieving a clear color display.
An object of the present invention is to provide a coloring material exhibiting an excellent durability and capable of a satisfactory color display.
Another object is to provide a coloring material for a color filter exhibiting an excellent durability, capable of a satisfactory color display, and capable of preventing the decomposed substances thereof from lowering the resistance value of the liquid crystal material.
Another object of the present invention is to provide a display device capable of a satisfactory color display.
Still another object of the present invention is to provide a liquid crystal display device that does not bring about a marked reduction in the resistance value of the liquid crystal material and capable of achieving a satisfactory color display.
According to a first aspect of the present invention, there is provided a coloring material comprising a plurality of organic basic skeletons each performing an acceptor function and a donor substituent serving to connect two organic basic skeletons, wherein color is developed by the intramolecular electron migration between the substituent and the basic skeleton.
According to a second aspect of the present invention, there is provided a coloring material for a color filter, comprising a plurality of organic basic skeletons each performing an acceptor function and a donor substituent serving to connect two organic basic skeletons.
According to a third aspect of the present invention, there is provided a display element comprising the coloring material described above.
Further, according to a fourth aspect of the present invention, there is provided a liquid crystal display device provided with a color filter containing the coloring material for the color filter.
It is desirable for the basic skeleton to be an anthraquinone-based structure.
It is desirable for the coloring material for the color filter to have a chemical structure represented by any of general formulas (1) and (9): 
where each of R1, R2 and R3, which may be the same or different, is at least one of hydrogen and an alkyl group, each of X1 and X2, which may be the same or different, is selected from the group consisting of S, O, NH and Se, and Z is at least one of hydrogen and an alkyl group; 
where R1, R2, R3, R4, A and B is selected from the group consisting of hydrogen atom, an alkyl group, a halogen atom, a nitro group, an amino group, a monoalkyl group, a dialkyl amino group, an alkoxy group, and a thioalkoxy group, and each of m and n is an integer of 1 to 4, wherein R1, R2, R3, R4 may be the same or different, and A and B may be the same or different.
When X1 and X2 of the formula (1) is S, the compound represented by the formula (1) is a di-sulfide.
It is desirable for the coloring material for a color filter to have a repeating unit represented by general formula (3) given below: 
where each of R5 and R6, which may be the same or different, is one of hydrogen atom and an alkyl group, and each of X1, X2, X3 and X4, which may be the same or different, is selected from the group consisting of S, O, NH and Se.
To be more specific, it is desirable for the coloring material for a color filter to have a chemical structure represented by general formula (2) given below: 
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.